Helping people with computers... one answer at a time.
Not being able to access certain web sites isn't that uncommon. Unfortunately there are many possible reasons.
Why can I not access certain web sites?
There are so many possible reasons, and it's hard to say with only that to go on. But I can throw out some possibilities.
If "certain sites" are sites that others might find objectionable, it's possible that there's a content filter in place; either on your computer or on some equipment between it and the internet. Libraries often use this type of technology to prevent minors from accessing pornographic web sites.
If "certain sites" are anti-virus manufacturers, then there's a good chance you have a virus yourself. Several are known to take steps to prevent you from downloading anti-virus software. (You'll have to use another computer to do that.)
If "certain sites" are all related - say all of the Yahoo sites are inaccessible for some reason, then it's possible that there's a transient problem at the remote end. Try again later.
If "certain sites" means that when you go to one site, you land on some other site you didn't want, then you probably have spyware on your machine. Get a scanner and scan.
Those are the common issues. There are certainly technical issues that can cause this type of thing, but since you can access some sites and not others, it's less likely to be a problem on your machine.
Update: Thanks to reader comments, it turns out that some folks issues can be resolved by adjusting the "MTU" setting on their internet connection. This article walks you through that process: I can't access some websites ... why?.
Update #2: another common reason for not being able to visit certain websites is detailed here: Why did my home network stop working?